1. Field of the Invention
The present invention relates to an image pickup apparatus such as a video camera and, more particularly, to an arrangement which is suitable for use in an apparatus using an inner focus type of lens system.
2. Description of Related Art
FIG. 2 is a view showing a simple arrangement of an inner focus type of lens system which has conventionally been used. The arrangement shown in FIG. 2 includes a fixed first lens group 101, a second lens group (variator lens) 102 for performing a magnification varying operation, an iris 103, a fixed third lens group 104, a fourth lens group (focusing lens) 105 having both a focus adjusting function and a so-called compensation function which compensates for a movement of a focal plane due to a-magnification varying operation, and an image pickup element 106.
As is already known, in the lens system which is arranged as shown in FIG. 2, since the focusing lens 105 has both the compensation function and the focus adjusting function, the position of the focusing lens 105 for forming an in-focus image on an image pickup surface of the image pickup element 106 differs for different subject distances even in the case of the same focal length. If a variation in the position of the focusing lens 105 for forming an in-focus image on the image pickup surface of the image pickup element 106 is continuously plotted against different subject distances for different focal lengths, the resultant loci are as shown in FIG. 3. During a magnification varying operation, zooming free of defocusing is enabled by selecting a locus from the loci shown in FIG. 3 according to the subject distance and moving the focusing lens 105 along the selected locus.
A front-lens focus type of lens system is provided with a compensator lens which is independent of a variator lens, and the variator lens and the compensator lens are connected to each other by a mechanical cam ring. Accordingly, if a knob for manual zooming is provided on the cam ring so that the focal length can be manually varied, no matter how fast the knob may be moved, the cam ring rotates in accordance with the movement of the knob, and the variator lens and the compensator lens move along a cam groove in the cam ring. Therefore, as long as the focusing lens is in focus, the above operation does not cause defocusing.
In the control of the above-described inner focus type of lens system, it is general practice to previously store a plurality of pieces of locus information such as those shown in FIG. 3 in a lens control microcomputer in a particular form, select a locus according to the relative position between the focusing lens and the variator lens, and perform zooming while tracing the selected locus. In such control, it is necessary to read the position of each of the focusing lens and the variator lens with a certain degree of accuracy, because the position of the focusing lens relative to the position of the variator lens is read from a storage element and applied to lens control.
As can be seen from FIG. 3 as well, if the variator lens moves at or near a uniform speed, the inclination of the locus of the focusing lens successively varies with a variation in the focal length. This indicates that the moving speed and direction of the focusing lens vary successively. In other words, an actuator for the focusing lens, if it is a stepping motor, needs to make a highly accurate speed response of 1 Hz up to several hundred Hz.
It is becoming general practice to use a stepping motor for the focusing lens group of the inner focus type of lens system as an actuator which satisfies the above requirement. The stepping motor is capable of rotating in complete synchronism with a step pulse outputted from a lens control microcomputer or the like and showing a constant stepping angle per pulse, so that the stepping motor can realize high speed response, high stopping accuracy and high positional accuracy. Furthermore, the stepping motor provides the advantage that since its rotating angle per step pulse is constant, the step pulse can be used for an increment type of encoder and a special position encoder is not needed.
As described above, if a magnification varying operation is to be carried out while keeping an in-focus state by using such a stepping motor, it is necessary to previously store the locus information shown in FIG. 3 in the lens control microcomputer or the like in a particular form (the loci themselves may be stored or a function which uses lens positions as variables may be stored), and read locus information according to the position or the moving speed of the variator lens and move the focusing lens on the basis of the read locus information.
FIG. 4 is a view aiding in explaining a locus tracing method which has previously been proposed. In FIG. 4, Z0, Z1, Z2, . . . , Z6 indicate the position of the variator lens, a0, a1, a2, . . . , a6 and b0, b1, b2, . . . , b6 respectively indicate representative loci stored in the lens control microcomputer, and p0, p1, p2, . . . , p6 indicate a locus calculated on the basis of the two loci. An equation for calculating this locus is shown below:p(n+1)=(|p(n)−a(n)|/|b(n)−a(n)|)×|b(n+1)−a(n+1)|+a(n+1)  (1)
According to Equation (1), for example, if the focusing lens is located at the point p0 in FIG. 4, the ratio in which the point p0 internally divides a line segment b0–a0 is obtained, and a point which internally divides a line segment b1–a1 in accordance with that ratio is determined as p1. The standard moving speed of the focusing lens required to keep an in-focus state can be found from the p1–p0 positional difference and the time required for the variator lens to move from Z0 to Z1.
A case in which the stop position of the variator lens is not limited only to boundaries having stored representative locus data will be described below with reference to FIG. 5.
FIG. 5 is a view aiding in explaining a method of interpolating the position of the variator lens. FIG. 5 is an extracted portion of FIG. 4 (a dashed-line portion in FIG. 4) and shows a case in which the variator lens can be stopped at an arbitrary stop position. In FIG. 5, the vertical and horizontal axes respectively represent the position of the focusing lens and the position of the variator lens. Letting Z0, Z1, . . . , Zk−1, Zk, . . . Zn represent the position of the variator lens, the corresponding positions of the focusing lens for different subject distances, i.e., the representative locus positions (the position of the focusing lens relative to the position of the variator lens) stored in a lens control microcomputer are as follows:
a0, a1, . . . , ak−1, ak, . . . an,
b0, b1, . . . , bk−1, bk, . . . bn.
If it is now assumed that the position of the variator lens is Zx which is not a zoom boundary position and that the position of the focusing lens is px, positions ax and bx are obtained as follows:ax=ak−(Zk−Zx)×((ak−ak−1)/(Zk−Zk−1)),  (2)bx=bk−(Zk−Zx)×((bk−bk−1)/(Zk−Zk−1)).  (3)Specifically, in accordance with an internal ratio which is obtained from the current position of the variator lens and two adjacent opposite zoom boundary positions (for example, Zk and Zk−1 in FIG. 5), locus data corresponding to the same subject distance are selected from among four stored representative locus data (ak, ak−1, bk, bk−1 in FIG. 5) and are internally divided by the internal ratio shown by the above equation (1), whereby ax and bx can be obtained.
Then, in accordance with an internal ratio which is obtained from ax, px and bx, the locus data corresponding to the same focal length, which are selected from among the four stored representative locus data (ak, ak−1, bk, bk−1 in FIG. 5), are internally divided by the internal ratio shown by the above equation (1), whereby pk and pk−1 can be obtained. Furthermore, during zooming from the wide-angle end toward the telephoto end, the moving speed of the focusing lens required to keep an in-focus state can be found from the difference between the target focusing-lens position pk and the current focusing-lens position px and the time required for the variator lens to move from Zx to Zk.
Furthermore, during zooming from the telephoto end toward the wide-angle end, the standard moving speed of the focusing lens required to keep an in-focus state can be found from the difference between the target focusing-lens position pk−1 and the current focusing-lens position px and the time required for the variator lens to move from Zx to Zk−1. The above-described locus tracing method has been devised.
As can be seen from FIG. 3, if the variator lens moves from the telephoto end toward the wide-angle end in the direction in which divergent loci gradually converge, an in-focus state can be maintained by the above-described locus tracing method. However, if the variator lens moves from the wide-angle end toward the telephoto end, it is impossible to determine which locus should be traced by the focusing lens which is located at a point on convergent loci, so that an in-focus state cannot be maintained by the above-described locus tracing method.
FIGS. 6(A) and 6(B) are views aiding in explaining one example of a locus tracing method which has previously been devised to solve the above-described problem. In each of FIGS. 6(A) and 6(B), the horizontal axis represents the position of the variator lens, and the vertical axis of FIG. 6(A) represents the level of a high-frequency component (sharpness signal) of a video signal which is an AF evaluation signal, whereas the vertical axis of FIG. 6(B) represents the position of the focusing lens.
In FIG. 6(B), it is assumed that a locus 604 is an in-focus cam locus to be used for zooming relative to a certain subject. It is also assumed that the standard moving speed for in-focus cam locus tracing on the wide-angle side of a zoom position 606 (Z14) is positive (the focusing lens moves toward its closest-distance end), and that the standard moving speed for in-focus cam locus tracing on the telephoto side of the zoom position 606 is negative (the focusing lens moves toward its infinity end). If the focusing lens traces the cam locus 604 while maintaining an in-focus state, the magnitude of the AF evaluation signal becomes as shown at 601 in FIG. 6(A). It is generally known that zooming which maintains an in-focus state exhibits an AF evaluation signal level which has an approximately constant value.
In FIG. 6(B), Vf0 indicates the standard moving speed of the focusing lens which traces the in-focus cam locus 604 during zooming, and Vf indicates an actual moving speed of the focusing lens. If zooming is performed while varying its speed with respect to the speed Vf0 which traces the locus 604, a zigzag locus like a locus 605 is obtained. In this case, the sharpness signal level varies in such a manner that hills and valleys repeatedly occur like a locus 603.
The magnitude of the sharpness signal 603 reaches its maximum at each position where the loci 604 and 605 cross each other (even-numbered points among Z0, Z1, . . . , Z16), whereas the magnitude of the sharpness signal 603 reaches its minimum at each position where the moving-direction vector of the locus 605 switches over (odd-numbered points among Z0, Z1, . . . , Z16). The sharpness signal 603 has a minimum value 602, and if the minimum value 602 is set as a level TH1 and the moving-direction vector of the locus 605 is switched over each time the magnitude of the sharpness signal 603 becomes equal to the level TH1, the moving direction of the focusing lens after switchover can be set to a direction closer to the locus 604.
In other words, each time an image is defocused by the difference between the levels 601 and 602 (TH1) of the AF evaluation signal, if the moving direction and the moving speed of the focusing lens are controlled to decrease the amount of defocusing, it is possible to effect zooming with the amount of defocusing reduced.
By using the above-described method, in the case of zooming from the wide-angle end toward the telephoto end in which convergent cam loci gradually diverge as shown in FIG. 3, even if the standard moving speed Vf0 of the focusing lens which maintains an in-focus state is not optimum for a target subject distance, it is possible to select a locus capable of preventing the AF evaluation signal level from falling below the minimum value 602 (TH1), i.e., preventing occurrence of not less than a certain amount of defocusing, by repeating a switchover operation like the locus 605 in accordance with a variation in the AF evaluation signal level while controlling the moving speed Vf of the focusing lens with respect to the standard moving speed (calculated by using p(n+1) obtained from Equation (1)). Furthermore, regarding the amount of defocusing, if the level TH1 is appropriately set, it is possible to realize zooming during which defocusing apparently is not observed.
Letting Vf+ and Vf− be a positive correction speed and a negative correction speed, respectively, the moving speed Vf of the focusing lens is determined as:Vf=Vf0+Vf+,  (4)Vf=Vf0+Vf−.  (5)At this time, to prevent the correction speeds Vf+ and Vf− from deviating in either correction direction when a focus locus to be traced is selected, the correction speeds Vf+ and Vf− are determined so that the internal angle made by the two direction vectors of the moving speed Vf which are obtained from the above equations (4) and (5) is divided into two equal angles by the direction vector of the standard moving speed Vf0. In addition, another method has been devised which is intended to improve the accuracy of selection of a focus locus to be traced, by varying the increase-decrease period of the sharpness signal by varying the amount of correction due to a correction speed according to the kind or state of a subject, the focal length or the depth of field.
In general, the above-described control for the magnification varying operation is performed in synchronism with a vertical synchronizing signal because a video signal from an image pickup element is used to detect focus.
FIG. 7 shows a control flowchart of a conventional example of lens control performed by a lens control microcomputer. Step S1 indicates the start of processing. Step S2 is an initial setting routine for executing the processing of initializing various ports and a RAM in the lens control microcomputer.
Step S3 is a routine for intercommunication with a system control microcomputer which controls the operating system of a camera body. In Step S3, when the lens control microcomputer receives zoom-switch-unit information from a zoom switch unit operated by a photographer, the lens control microcomputer provides magnification-varying-operation information, such as the position of a zooming lens, to inform the photographer that a zooming operation is being executed, and the information is given to the photographer through a display or the like.
Step S4 is an AF processing routine for performing the processing of making automatic adjustment of focus according to a variation in the AF evaluation signal.
Step S5 is a zooming processing routine for processing a compensation operation for maintaining an in-focus state during a magnification varying operation.
By the above-described method, calculations are performed on a standard driving direction and a standard moving speed of a focusing lens which traces a cam locus such as that shown in FIG. 4.
Step S6 is a routine for making selection from among the driving directions and the driving speeds for the variator lens and the focusing lens which have been calculated in the processing routines of Steps S4 and S55, according to whether to execute an AF operation or a magnification varying operation, and executing setting so as not to drive the lenses beyond their respective telephoto ends, wide-angle ends, closest-distance ends or infinity ends all of which are set by software so as not to prevent the lenses from coming into contact with end portions of their respective mechanical portions.
In Step S7, the lens control microcomputer outputs control signals to motor drivers according to the driving directions and the driving speeds for the variator lens and the focusing lens which have been determined in Step S6, thereby controlling the respective motors to drive or stop the variator lens and the focusing lens.
After the completion of the processing of Step S7, the process returns to Step S3.
The entire processing shown in FIG. 7 is executed in synchronism with each vertical synchronizing period (in the processing of Step S3, the process waits for the arrival of the next vertical synchronizing signal).
However, in the case of a recent type of video camera having a far faster zooming speed, for example, the variator lens may often move from a position Z4 to a position Z6 (shown in FIG. 4) within the time of one vertical synchronizing period. During this time, if the lens control processing of FIG. 7 is performed in synchronism with the vertical synchronizing period, the standard moving speed of the focusing lens remains the speed at which the focusing lens is moving from p4 to p5, and the updating of the standard moving speed is not performed until the variator lens reaches the position Z6. Accordingly, when the position of the variator lens is Z6, the focusing lens lies at a point p6′ on a line which rectilinearly extends from the line p4–p5 in FIG. 4, so that defocusing occurs by the difference between p6′ and p6 and accurate tracing of a cam locus cannot be performed during zooming.
To solve the above-described problem, a method based on the processing routine shown in FIG. 8 has been proposed. In this method, the standard moving speed of a focusing lens is calculated by a plurality of times (twice, in the example shown in FIG. 8) within one vertical synchronizing period so that the occurrence of defocusing is prevented. In FIG. 8, the processing of Steps S11 to S17 is similar to that of Steps S1 to S7 of FIG. 7.
After the completion of the processing of Step S17, the process waits for a predetermined period of time in Step S18 until the middle point of the vertical synchronizing period. After the lapse of the predetermined time, if it is determined in Step S19 that zooming is being executed, it is determined that the position of the variator lens has been updated, and processing similar to the processing of Steps S15 to S17 is again executed.
In Step S20, the driving directions for the variator lens and the focusing lens as well as the standard moving speed for the focusing lens are again calculated, and in Step S21, selection is made from among the driving directions and the driving speeds for the variator lens and the focusing lens which have been calculated in Step S20. In Step S22, the selected driving directions and speeds are output to the respective motor drivers to execute lens control, and the process then returns to Step S13.
If it is determined in Step S19 that zooming is not being executed, the process returns to Step S13 and waits for the next operation.
The entire processing shown in FIG. 8 is executed in synchronism with the vertical synchronizing period, and in the processing of Step S13, the process waits for the arrival of the next vertical synchronizing signal.
If the standard moving speed of the focusing lens is calculated only once within one vertical synchronizing period during zooming, the focusing lens reaches the point p6′ at a focusing speed equivalent to the inclination of the line p4–p5 during the movement of the variator lens from Z4 to Z6 (in FIG. 4) within one vertical synchronizing period. In contrast, in the above-described method, since the standard moving speed of the focusing lens is calculated twice within one vertical synchronizing period, the focusing lens reaches the point p5 at a focusing speed equivalent to the inclination of the line p4–p5 during the first half of one vertical synchronizing period, and moves past the point p5 at a focusing speed equivalent to the inclination of the line p5–p6 during the second half of the one vertical synchronizing period, so that the focusing lens can reach the point p6 after the one vertical synchronizing period. Accordingly, it is possible to realize accurate tracing of a cam locus and prevention of occurrence of defocusing.
However, in the above-described conventional example, since the standard moving speed of the focusing lens is calculated by a plurality of times during one vertical synchronizing period so that defocusing is prevented during the tracing of a cam locus, the load on the lens control microcomputer becomes large during high-speed zooming. Specifically, the conventional example needs a microcomputer having a fast processing speed which is capable of executing a calculation of the standard moving speed by a plurality of times during one vertical synchronizing period, and a video camera using such a microcomputer becomes expensive for a user.
The standard moving speed for the focusing lens which is calculated by the above-described cam locus tracing method is obtained by calculating a destination target position relative to a zoom-lens position having representative locus data indicative of the closest distance to the current zoom position Zx, i.e., the boundary position (Zk−1 or Zk) in the zoom area shown in FIG. 5. Accordingly, there is a case in which the period of time required for the variator lens to move from Zx to Zk−1 or Zk is short because of the timing of executing the calculation. At this time, a large calculation error occurs in the division computation ((the moving distance of the focusing lens)÷(the time period of movement of the variator lens)) required to calculate the standard moving speed, so that the problem that an in-focus locus cannot be accurately traced also arises.
A number of problems which occur in the above-described conventional cam locus tracing method will be further described below with reference to an example in which a linear motor is used as a lens driving actuator. Linear motors have recently been used in more and more products because of their superior high-speed performance.
In general, in a system in which a linear motor such as a voice coil motor is used as a focusing motor, a position encoder for detecting the position of a focusing lens is disposed to form a feedback loop so that a deviation signal between the output signal of the position encoder and a target position signal outputted from a control circuit approaches zero, and the driving speed of the motor is determined by the response characteristics of the feedback loop.
Accordingly, the focus correcting operation of the focusing lens during the tracing of a cam locus is effected not by a control method based on the driving direction and the driving speed but by a control method using a destination target position as a parameter. Accordingly, during the tracing of a cam locus, the destination target position to be reached by the focusing lens corresponds to the position px obtained from the above-described equation (1).
However, in a recent type of video camera having a far faster zooming speed, for example, in a case where the variator lens moves from the position Z4 to the position Z6 (shown in FIG. 4) within the time of one vertical synchronizing period, if the lens control processing of FIG. 7 is performed in synchronism with the vertical synchronizing period as in the case of the above-described conventional example, the point p5 at a zoom boundary having cam locus data is calculated as a target position. Even if the variator lens proceeds to Z6, the updating of the target position is not performed, and because of loop control, the position of the focusing lens remains p5 (p6″ in FIG. 4) and defocusing occurs.
In particular, both the time required for the variator lens to move by the distance difference between the current position of the variator lens and the zoom boundary position and the time required for the focusing lens to move by the distance difference between a calculated target trace position and the current position of the focusing lens vary depending on computation timing and zooming speed. Accordingly, if the focusing lens is to be located at a target position when the variator lens reaches a boundary, it is necessary to execute complicated processing extremely difficult to realize.